1. Field of the Invention
The present invention relates to an electromagnetic actuator that can be suitably used for active vibration damping devices such as a vibration damping mount, damper or the like that exhibits an active damping effect.
2. Description of the Related Art
In order to reduce vibration in members subject to vibration damping such as an automobile body or the like for which vibration reduction is viewed as important, there are typically employed vibration damping devices such as vibration damping means utilizing attenuation effect of a shock absorber, a rubber elastic body or the like, and vibration insulation means utilizing spring effect of a coil spring, a rubber elastic body or the like. These known vibration damping devices exhibit passive vibration damping effects, so that these known devices are not able to exhibit desired vibration damping effect when frequency characteristics of vibration to be damped varies, or when a higher level of vibration damping effect is required. In light of this, typically used are active vibration damping devices, which are designed to apply excitation or oscillation force to members subject to vibration damping, thereby attenuating vibration in the subject member in an active or cancellation fashion.
The aforementioned active type vibration device needs an actuator for generating oscillation force. To obtain an excellent vibration damping effect, it is important to control the actuator with high precision in terms of frequency and phase of the generated oscillation force. To meet this end, an electromagnetic actuator may be effectively used for the actuator for generating the oscillation force. This electromagnetic actuator typically includes: a stator having a coil with a central through hole and a yoke member disposed around the coil to form a magnetic path; and a movable member having an oscillation rod disposed extending through the through hole of the coil while being movable relative to the stator in the axial direction. Upon energizing the coil, drive force is generated in the axial direction between the stator and the movable member.
When a large impact or the like acts as the external force on the active vibration damping device equipped with this kind of electromagnetic actuator, the movable member may displace excessively, possibly striking against the housing or the like. This striking of the movable member may possibly cause undesirable noise or impacts, leading to undesirable noise or damage of actuator itself.
In order to eliminate the problems of this kind of striking of the movable member on the housing or the like, for example, U.S. Pat. No. 7,066,454 and JP-A-2005-172202 or other documents propose a constitution wherein a rubber sheet for shock absorption is placed on the inner surface of the housing arranged facing the oscillation rod of the electromagnetic actuator. However, this kind of simple rubber sheet may not realize sufficient shock absorption, when a large shock load is applied on the active vibration damping device. Further, the repeated striking of the movable member against the housing may deteriorate the durability of the rubber sheet, leading to damage of rubber sheet it self.
Furthermore, due to reasons of the actuator assembly steps or reasons such as the work when doing various adjustments on the actuator, such as adjustment of the oscillation rod axial direction position or the like, an adjustment hole that pierces through in the axial direction is formed at the portion at which the rubber sheet is placed on the electromagnetic actuator. Through this adjustment hole, if foreign matter such as dust or water or the like penetrates from the outside, smooth excitation displacement of the movable member is deterrent, making it impossible to obtain the initially generated oscillation force, or leading to a deterioration of durability or the like due to erosion or the like. Therefore, it is important that the opening of the adjustment hole is closed fluid-tightly. To meet this end, U.S. Pat. No. 7,066,454 discussed above teaches an installation of an O ring for sealing at part of the outer peripheral surface of a lid member (stopper) fixed by screwing inserted within the through hole of the coil, thereby providing a fluid tight sealing between the lid member and the inner peripheral surface of the through hole. However, this kind of sealing constitution needs a separate sealing rubber, thereby increasing the number of components, possibly leading to a problem of a reduction in productivity and the like.